Artistic plagiarism

Has anyone else noticed how many films are based on something else?
Not just the bad films either - a whole bunch of really good movies are adaptations of something else. For example: Citizen Kane was based on the life on William Randolph Hearst, Ben Hur is based on a book, as was The Lord of the Rings, The Godfather, 2001: A Space Odyssey and The Wizard of OZ. Furthermore, Schindler's List, Titanic, Saving Private Ryan and Patton are all based on historical events (and three of them are based on the Second World War).
Then you have hybrids. Apocalypse Now is both an adaptation of the book Heart of Darkness, and an interpretation of the Vietnam War. The books that both Psycho and Silence of the Lambs were based on were inspired by the serial killer Ed Gein. West Side Story was an adaptation of a Broadway musical that itself was inspired by Romeo and Juliet, and speaking of which, I won't even hazard a guess of how many times Shakespeare has been adapted for film.
Getting to the point, even a casual glance at any "best of" list will reveal at least a handful of movies that started out as another medium. This begs the question as to why so many works are adapted from elsewhere, and why these films are still considered popular. I believe that the answer lies with the movie Star Wars.
When George Lucas created Star Wars Episode IV: A New Hope, he drew from a mass of sources, both literary and cinematic. For example, C3P0's look is eerily similar to the robot from Fritz Lang' Metropolis, and the look of Tatooine looks strikingly like the eponymous planet from Dune. However the most important influence is clearly Joseph Campbell's book, The Hero with a Thousand Faces.
In his book, Campbell describes his theory of "The Monomyth" - a term that he borrowed from James Joyce. Those of you who remember High School English will know this by another name: The Hero's Journey. The theory is that the monomyth's structure comes from a basic need that all humans have for a certain kind of story. These stories then become the great cultural myths that are retold over and over again.
Many myths from many cultures follow this pattern, and many heroes have taken the journey. Campbell gives the examples of The Buddha, Christ and Gilgamesh. However, we can extrapolate the term to include Superman, Harry Potter and Luke Skywalker.
Getting back to Star Wars, Lucas based his movie off of the heroes' journey; it became insanely popular, and earned him a tonne of money. Campbell's influence is no secret, there's even a quote from George Lucas on the back of the most recent edition of Thousand Faces. Star Wars then pretty much defines what I'm trying to get at.
Essentially, people need to retell their stories. That idea is the reason for the monomyth, it's why Star Wars made so much money, and it's why they made another film about Alice in Wonderland.
It's not the content - content is largely irrelevant after a certain point. Stories like Titanic or Citizen Kane are so well loved because they tapped into our collective unconscious (to borrow a Jungian term), and held it over a long period of time. That's why The Odyssey is still relevant after many thousands of years.
If the story is a good one, we'll never want to stop hearing it, especially if it gets remade with Tim Burton and Johnny Depp.